diff -r 149c5af35de5 -r 6e90c59b6da1 mercurial/commands.py
--- a/mercurial/commands.py	Thu Mar 01 23:52:30 2018 +0530
+++ b/mercurial/commands.py	Thu Mar 01 23:54:52 2018 +0530
@@ -2370,7 +2370,7 @@
     try:
         regexp = util.re.compile(pattern, reflags)
     except re.error as inst:
-        ui.warn(_("grep: invalid match pattern: %s\n") % inst)
+        ui.warn(_("grep: invalid match pattern: %s\n") % pycompat.bytestr(inst))
         return 1
     sep, eol = ':', '\n'
     if opts.get('print0'):